1. Field of the Invention
This invention relates to a shape memory alloy actuator utilizing a shape restoring force of a shape memory alloy (SMA) and a method of designing the same.
2. Related Art
Generally, in conventional shape memory alloy actuators, their shape memory alloys tends to lose the memory of the given configuration, and permanent strains tend to be produced in the alloy in a short period of time. Therefore, the conventional shape memory alloy actuators have short lives and unstable performances, and their shape memory alloys are unstable in configuration. Besides, they utilize only a very limited range of recoverable strains of the shape memory alloys and thereby their operating ranges are limited.
It is accordingly an object of the present invention to provide a shape memory alloy actuator that withstands a large number of operation cycles and has a far longer service life than the conventional shape memory alloy actuators and a method of designing such a shape memory alloy actuator.
It is another object of the present invention to provide a shape memory alloy actuator that is stable in configuration of its shape memory alloy and a method of designing such a shape memory alloy actuator.
It is still another object of the present invention to provide a shape memory alloy actuator that is stable in performance and a method of designing such a shape memory alloy actuator.
It is a further object of the present invention to provide a shape memory alloy actuator that can effectively utilize almost the whole range of recoverable strain of its shape memory alloy and has a far wider operating range than the conventional shape memory alloy actuators and a method of designing such a shape memory alloy actuator.
It is a still further object of the present invention to provide a shape memory alloy actuator with respect to which the required amount of shape memory alloy can be estimated very easily and a method of designing such a shape memory alloy actuator.
The present invention employs a shape memory alloy exhibiting a two-way shape memory effect and having a stress-strain property that, in a stress-strain diagram with the stress plotted in ordinate, the upward direction taken as the positive direction, and the strain plotted in abscissa, the rightward direction taken as the positive direction, the stress-strain curve at a low temperature comprises a gentler gradient portion extending from the position at which the strain is zero to the right with relatively small gradients, in the area in which the stress is negative or approximately along the line on which the stress is zero, and a steeper gradient portion extending upward to the right with relatively great gradients on the right of said gentler portion. The present invention operates the shape memory alloy in the region surrounded by the gentler gradient portion, the steeper gradient portion, a practical stress limit line on which the stress in the shape memory alloy reaches a specified practical limit, the straight line connecting the intersection of the practical stress limit line and a strain limit line near shape recovery completion on which the strain of the shape memory alloy reaches a specified value in a state close to the shape recovery completion and the point where the strain is zero at a low temperature in the stress-strain diagram.